1. Field of the Invention
The present invention relates to a silhouette cutting apparatus for cutting a peel-off film in correspondence with a silhouette of a necessary portion so as to form a mask in order to mask an unnecessary portion while leaving the necessary portion used for printing a color separation film in color printing and, more particularly, to a technique of detecting silhouette information of the necessary portion.
2. Description of the Related Art
When a color separation plate for color printing is formed, a color scanner is used. The color scanner photoelectrically reads a color original, separates image signals into color signals in units of color components, and exposes a color separation film accordingly. The film is developed to form a color separation plate.
The color scanner can designate an area of color separation in the color original. However, the designated area is limited to a rectangular range. For this reason, when only a person or commodity is printed like a leaflet without using an entire color original for printing and a background color is required to be white or a predetermined color, an unnecessary portion (background) of a color separation plate must be masked when a positive original plate for printing a press plate is formed from the color separation plate. In order to form the positive original plate for printing the press plate from the color separation plate, a photographic technique such as contact printing is adopted. In this technique, the color separation plate is locally shielded from printing light to achieve masking. For this purpose, a peel-off film prepared by adhering a red sheet on a transparent sheet is cut along a silhouette of a necessary portion.
The peel-off film from which the red sheet corresponding to the necessary portion is cut is overlaid on the color separation plate so as to form a positive original plate for printing the press plate, thus shielding the unnecessary portion.
Conventionally, the mask is formed by overlaying the peel-off film on the color separation plate and cutting it along the silhouette by a skilled person. However, this cutting requires considerably high precision and much time. In addition, satisfactory ones cannot often be obtained. If this mask is offset from the silhouette of an image, the silhouette of the image is indented or becomes large, or the image is deformed, resulting in poor appearance.